AEV-type premix burners (AEV=Advanced Environmental Vortex) are used in stationary gas turbine plants. A premix burner of this kind is disclosed by EP-0 704 657 A2, and said publication forms an integral part of the present application. The same applies to the additional developments of this premix burner for which applications have been made since then, e.g. EP-0 913 630 A1, U.S. Pat. No. 6,045,351, WO-A1-2006/048405. To aid comprehension, the explanation of the situation with reference to FIG. 1 of the present application will be kept such that the reference signs are also included. The premix burner 11 shown in FIG. 1 is fitted at the head end with a swirl generator and, on the downstream side, comprises a mixing tube 13 which, for its part, opens into a combustion chamber 14. This burner arrangement 10 extends along an axis 15. The air/fuel mixture 12 produced in the premix burner 11 is injected tangentially between the conical component shells, leading to the formation of a swirling flow which then undergoes complete mixing in the downstream mixing tube 13 with the fuel F introduced. Passing the fuel 25 exclusively to the swirl generator is not the only possibility: if required, a partial quantity 25′ of the fuel F is introduced into the mixing tube 13.
An additional component of this burner arrangement 10 is a film air ring 16 arranged in the mixing tube 13 (indicated by the dashed box in FIG. 1), the purpose of which is to energize the flow layers near to the wall in the mixing tube to such an extent that the fuel located in this region or adhering there cannot cause backfiring.
As can be seen from the abovementioned publication (see FIG. 1 of that document), two annular rows of holes are arranged axially in series in the known film air ring, said holes meeting certain requirements in terms of their number, diameter and spatial orientation. By means of these rows of holes, a predetermined air throughput is achieved, amounting approximately to 1% of the total air throughput of the burner.
If fuels other than natural gas are burned in such a burner arrangement, the fuel is often trapped close to the wall of the mixing tube 13 in zones of low flow velocity. This then causes backfiring into the premix burner 11:                In the case of dry oil: high-pressure combustion tests under certain operating conditions have shown that oil strikes the walls of the mixing tube.        In the case of H2-rich fuels: LIF tests and combustion tests at 1 atm have confirmed that the comparatively low density of these fuels (especially as they are at relatively high temperatures, typically 150-350° C.) means a low penetration depth and a high risk that the fuel will readhere to the walls of the mixing tube.        
Experience in the development of burners for dry oil and H2-rich fuels has indicated that it might be useful to increase the film air quantity in order to further energize the boundary layers near to the wall (i.e. to increase the local flow velocities) and thus avoid the risk of backfiring. Another factor contributing to this result is that the fuel concentration near to the wall is minimized. Normally, this could be achieved by increasing the cross section of the holes in the film air ring 16 (i.e. by boring the holes to a larger diameter). However, this leads to more powerful air jets, which penetrate the main flow in the mixing tube and no longer generate a film near to the wall. This is counterproductive for two reasons:                The boundary layer near to the wall is no longer energized, and the lower local velocities that result therefore promote the occurrence of backfiring.        The more powerful air jets penetrate the main flow and generate severe turbulence and a turbulent wake. Fuel caught in these vortices has a greatly prolonged dwell time in the mixing tube owing to the low local velocities there, and is correspondingly easy to ignite and can thus lead to backfiring.        